One system wherein food is stored over a wide temperature range in a multilayer thermoplastic film-type casing under conditions of substantial physical abuse and impact is the Capkold System marketed by the Groen Division of the Dover Corporation. Capkold is designed as a food preparation, storage and distribution system for food service operations as for example in restaurant chains, hospitals and schools.
In the Capkold System, large quantities of food as for example meat and poultry products are cooked at temperatures of at least 180.degree. F. (82.degree. C.) and then pumped at this temperature into the casing. The processed food-containing casing package is closed, as for example by clipping or heat sealing, and tumble-cooked to about 40.degree. F. (4.degree. C.) in a cold-water bath. Next the partially cooled food product package is transferred to a refrigeration zone for further cooling to 28.degree.-32.degree. F. (-2.degree.to 0.degree. C.) and normally stored for a sustained period which can be up to 45 days. During this refrigerated storage period the food package may for example be transferred from a central processing location to a number of food consumption sites at remote locations. When needed, the refrigerated (but not frozen) product packages may be reheated to the desired dispensing temperature as for example 120.degree. F. (49.degree. C.), the casing is opened and the contents emptied therefrom. The empty casing is discarded. Alternatively, the refrigerated product is transferred from the casing to another container for reheating.
There are a number of specific requirements for multilayer film useful as the Capkold casing material. It must include a core layer which provides an oxygen transmission rate below about 90 cc/M.sup.2 /24 hrs./atm. through the entire multilayer film. This is necessary to avoid spoilage of the enclosed food due to oxygen passage from the environment through the film wall. This requirement must be satisfied over the entire temperature range from hot filling at temperatures of at least about 180.degree. F. (82.degree. C.) to the refrigeration temperature of about 28.degree. F. (-2.degree. C.).
Another requirement is that the film must remain perfectly intact, ie. without any delamination between the core layer and both the inner and outer layers over the entire temperature range.
A further requirement is that the inner layer must have abuse resistance to withstand rigors of shipping and handling. Also, the inner layer must be completely chemically inert and nonreactive with the processed food over the temperature range of at least about 180.degree. F. to about 28.degree. F.
A still further requirement is that the outermost layer, generally termed the abuse layer, must be cross-linkable, withstand damage from dropping, and resist puncture resulting from impact during the tumble-cooling. Finally, the overall film must provide adequate tensile strength.
The prior art has employed multilayer films with at least five layers to satisfy these requirements of the Capkold System. For example, one such film broadly described in U.S. Pat. No. 4,104,404 to Bieler et al is marketed by W. R. Grace's Cryovac Division under the designation C300, and comprises an irradiated five layer film of polyethylene/adhesive/nylon/adhesive/polyethylene. One reason for the adhesive layers is that the presently used nylon core layer and the polyethylene outer layers do not bond well by direct adhesion.
According to the Bieler et al Patent, it is necessary to irradiate the five layer films at a relatively high dosage of at least 6 Mrad to provide the needed abuse resistance and delamination resistance.
It will be apparent from the foregoing that the presently used Capkold System films are complex both in terms of multiple materials and sophisticated manufacturing techniques, and expensive to manufacture.
An object of the present invention is to provide an improved multilayer thermoplastic film suitable for use in the Capkold System, which has less than five layers.
Another object is to provide a three layer film meeting all of the functional requirements for the Capkold System.
Still another object is to provide a three layer film with a nylon-type oxygen barrier core layer and outstanding adhesion properties with the outer layers over a wide temperature range between at least about 180.degree. F. and about 28.degree. F.
A further object is to provide a nylon core-type three layer film which not only has the aforedescribed outstanding adhesion properties but also affords good resistance to the physical abuse from cold water tumbling of a food package employing this film.
An additional object is to provide a nylon core-type three layer film which has all of the aforementioned properties without the need for a relatively high irradiation dosage on the order of 6 Mrad.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.